1. Field of the Invention
The present invention relates to energy conversion devices and more particularly to thermoelectric cells.
2. Related Art
Thermoelectric cells are well known in the prior art, for the conversion of electric power to heat or to convert heat to electric power. These cells, whose operation is based on the Seebeck effect or the Peltier effect, are used in their simplest form to measure temperature (as in many thermocouples) and in more complex structures to pump heat between a cold and hot reservoir, when an external electrical power is supplied, or to generate electrical power when an external thermal gradient is provided.
An elementary thermocouple consists of two dissimilar materials connected electrically at one end and having a thermal gradient between their connected ends and their respective opposing ends. Such a thermal gradient induces a voltage which varies with the thermal gradient imposed and depends on the relative electronic properties of the materials of the thermocouple (the Seebeck effect). Conversely, when a voltage is applied to the thermocouple it causes a thermal gradient to appear whose direction depends on the polarity of the applied voltage (the Peltier effect). Heat pumps using the thermoelectric effect usually involve two different semiconductor materials, one a p-type semiconductor (conductivity due to positive charge carriers or holes) and the other an n-type semiconductor (current carried by negative charge carriers or electrons). It is preferred that these semiconductors are capable of sustaining a large thermal gradient and therefore materials having low thermal conductivity are chosen. Typical materials used in thermoelectric cells are bismuth telluride (p and n type), lead telluride and various alloys of silicon and germanium.
When the thermoelectric cells are used as heat pumps or to transfer heat from a cold reservoir to a warmer reservoir, several thermocouples ("couple") are connected in series. Namely, the hot end of the p "leg" of one couple is connected to the hot end of the n "leg" of the next couple. Since all the cells are equal in composition, and dimensions, the voltage drop on each couple is the same (the total voltage divided by the number of cells) and a single thermal gradient is developed on the assembly between the cold junctions (between each pairs' cold legs) and the hot junctions (between neighboring pairs). Examples of such products are well known in the prior art and available for instance from Thermoelectron Corporation of Waltham Mass.
In the prior art, thermoelectric cells for heat pumps and for power generation units have been built in a planar geometry. Namely a multiplicity of thermoelectric cells are assembled between two planes with all the intercouple junctions on one plane and all the intracouple (between the two members of a couple) junctions on the opposing plane. Heat is transferred from one plane to the other plane when an appropriate DC voltage is applied to the assembly. The maximum temperature gradient achievable with a given couple depends on the properties of materials used in the couple. The temperature gradient of such planar devices can be further increased by cascading a number of devices in series thermally (but insulated electrically), so that the hot side of one device serves as the thermal cold side of the next device in the cascade. The heat pumping capacity can be increased by connecting additional devices thermally in parallel.
The planar structure of the thermoelectric cells of the prior art imposes a limitation on the quantity of heat that can be extracted through the cells' cold face. With a given type of thermocouple, and within the limitations imposed by maximum current that can be passed through such couples, only an increase in the cold surface area (and thus additional thermocouples) can increase the rate of heat extraction from the cold reservoir. Thus, when the device to be cooled is relatively small, and the heat that needs to be extracted from that device is large, a thermoelectric cell cannot be used. Furthermore, in a number of applications it is desired to cool a cylindrical core, for instance, laser rods or other cylindrical laser assemblies. In yet other applications it might be desired to heat a central solid core, or to heat or cool fluid flowing within a hollow core. All these cannot be satisfactorily achieved with the current state of the art thermoelectric cells.